In German Patent Application P 42 39 234.9 corresponding to U.S. patent application Ser. No. 08/446,836 filed 16 May 1995 now U.S. Pat. No. 5,693,408, a tool of the initially stated type is described in which the surface coating is comprised of a plurality of layers deposited in an alternating sequence by a thermal CVD process and by a glow discharge activated CVD process also known as plasma chemical vapor deposition (PCVD). The total thickness of the coating amounts to 2 through 40 .mu.m whereby each individual layer is thicker than 0.2 .mu.m.
The individual layers are applied by technically different processes. In the (thermal) CVD process one must use temperatures in excess of 900.degree. C. and pressures above 10,000 Pa to achieve sufficiently high coating rates. Thus, in an example of the aforementioned patent application, a gas mixture of 2.5% TiCl.sub.4, 35% N.sub.2 and 62.5 H.sub.2 is passed over the substrate to be coated at a temperature of 940.degree. C. and a pressure of 25000 Pa. Thereafter or alternatingly thereto, individual layers are applied by the PCVD processes whereby, for example, a gas mixture of 1.5% TiCl.sub.4, 11.5% N.sub.2, 8% Ar and 79% H.sub.2 is used at a pressure of 300 Pa and a temperature of 780.degree. C. The multilayer coating of CVD layers and PCVD layers gives an unusually good mechanical stability to the coating which yields a high useful life for tools fabricated by the process reviewed in chip removal operations. The special advantage of the alternating coating by means of the CVD process and PCVD process is that in a single coating apparatus in a continuous process without intervening coating and removal of the object to be coated, the coatings can be applied.